1. Field of the Invention
The present invention relates to a sheet depositing device for a sheet processing apparatus, comprising a feeding section for sequentially feeding sheets or sets of sheets from the sheet processing apparatus, at least one sheet stacking element movable along a guide, for facilitating the deposition of the sheets or sets of sheets fed by the feeding section in the sheet depositing device, and a sensor arrangement for detecting the position of a sheet stacking element along the guide, wherein the sensor arrangement includes a stationary linear array of active elements and a passive element moving in unison with the sheet stacking element.
2. Discussion of Background Art
An array of active sensors for determining a sheet stacking tray is known from U.S. Pat. No. 6,318,718 B1. This document is directed to a printer having a copy stacking tray that can be lowered to accommodate more sheets, and at the same time keeping the upper end of the sheet stack close to the sheet ejecting port of the print engine. Since the load of sheets on the tray increases with the tray position, the motor that moves the tray is provided with a gear box. A number of sensors are positioned along the path of the tray. When the tray passes one of the sensors, the gear ratio is adjusted for that position. However, the sensors are placed relatively far apart so that the arrangement is thus not suitable for a continuous tray position determination. No information as to the kind of sensors used is given.
U.S. Patent Application Publication No. US 2001/054791 A1 is directed to a printer in which the height of a sheet stack on a movable tray is determined by lowering a flat element onto the stack. In one embodiment, the exact position of the flat element is determined using a row of optical sensors.
Optical sensors have the advantage that they need no physical contact with the object they sense. However, a disadvantage of optical sensors is that they have an on/off behaviour, such that the resolution of the position determination is equal to the pitch of the optical sensors. If a high resolution is required, then many optical sensors must be mounted per mm, leading to high cost. Further, optical sensors are quite sensitive to dust pollution. In a sheet depositing device, dust, in the form of paper fibres, is omnipresent. Thus, sensor errors or failure are quite common when the optical sensors are used, the more so when small optical sensors are used to give a high resolution.